1. Field of the Invention
The present invention relates to a deposited film forming apparatus constructed to generate a plasma between a power applying electrode connected to a high frequency power source and a substrate disposed in opposition to the power applying electrode and serving as an electrode in a vacuum chamber to decompose a reactive gas introduced into the vacuum chamber to form a thin film as a deposited film on the substrate, and to a deposited film forming method using the deposited film forming apparatus.
2. Related Background Art
In the conventional deposited film forming apparatus, there are a flat plate type base member grounded and a power applying electrode disposed above the base member, housed in a vacuum chamber. The substrate serving as an electrode opposed to the power applying electrode in the vacuum chamber is guided into the vacuum chamber. The power applying electrode is electrically connected to a high frequency power source and the high frequency power source applies a power between the power applying electrode and the substrate. Then, a plasma is generated in a discharge space between the power applying electrode and the substrate to decompose the reactive gas introduced into the vacuum chamber and thus form a thin film as a deposited film on the substrate. In the ordinary plasma processing systems, the power applying electrode is fixed through an insulator to an internal wall of the vacuum chamber, for example, as disclosed in Japanese Patent Application Laid-Open No. 9-235676.
It was feasible heretofore to deposit thin films of amorphous semiconductors and the like on the substrate by use of the deposited film forming apparatus of this type. Since the amorphous semiconductors, e.g. amorphous silicon, can form a thin film in a small thickness and in a large area, provide a high degree of freedom for composition, and present controllability of electrical characteristics and optical characteristics within a wide range, they are recently drawing attention as materials for various semiconductor devices. Particularly, amorphous silicon is becoming a focus of attention as a material for solar cells, because it has such features that the absorption coefficient thereof is larger near the peak of the solar energy spectrum than those of crystalline silicon, that formation temperatures are low, and that a film can be directly formed from a source gas onto a substrate by glow discharge.
As for the solar cells increasingly valued as part of future new energy measures, reduction in cost and improvement in performance are significant subjects of research and development for the time being. Concerning the performance, solar cells with considerably high conversion efficiency have been yielded so far, but the reduction in cost of solar cells is not satisfactory yet. The reason is that the film forming rate of amorphous silicon is small.
A variety of proposals have been made heretofore as methods of forming a film of amorphous silicon at a high speed. An example is a method of decreasing the distance between the power applying electrode and the substrate, as disclosed in Japanese Patent Publication No. 5-56850.
During formation of a thin film of amorphous silicon or the like, the substrate, the power applying electrode, the discharge furnace, etc. are heated to a desired temperature in order to enhance the optical and electrical characteristics of the resultant thin film. Since electrons and ions accelerated by the plasma discharge collide with the substrate and power applying electrode, their temperatures increase. As a consequence, the substrate and power applying electrode undergo thermal expansion, which caused the substrate and power applying electrode to deform, e.g., warp, bend, or curve, as compared with their shapes set at room temperature.
The substrate undergoes no deformation or at most little deformation as long as the substrate is fixed to a substrate holder. The substrate holder is normally provided with a heater or the like and the substrate holder is considerably larger than the substrate. Therefore, the substrate holder is more resistant to deformation than the substrate. In the case of a belt-like substrate or the like guided into the discharge chamber in order to continuously convey the substrate in the roll-to-roll system without use of the substrate holder, the warpage and deformation of the substrate can be suppressed, for example, by securing the substrate by attraction of magnets at the edge portions of the substrate or by increasing the tensile force (tension) on the substrate. However, a thin film will inevitably be deposited on the power applying electrode, so that the power applying electrode tends to deform because of the stress of the thus deposited film. The conventional deposited film forming systems were not constructed in such structure as to hold the power applying electrode about to deform, and thus fine warpage or deformation was experienced.
Influence is little from the deformation of the power applying electrode where the distance between the power applying electrode and the substrate (electrode-substrate distance) is large. However, where the electrode-substrate distance is set small in order to increase the film forming rate, as disclosed in Japanese Patent Publication No. 5-56850, even fine deformation of the power applying electrode can cause a non-negligible range of influence on the electrode-substrate distance. As a result, the unevenness of the electrode-substrate distance resulted in nonuniformity of the plasma and partial difference of film forming rate, thereby posing a problem of unevenness of the film thickness. This problem would be a significant issue where the substrate is conveyed, particularly, where the film is formed while conveying the substrate in the roll-to-roll system.
Further, the deformation of the power applying electrode also makes the distance uneven between the power applying electrode and the base member supporting it, which can cause the plasma to intrude into the space (or gap) between them or which can induce an abnormal discharge in the space. It resulted in waste of the source gas, or generation of polysilane powder because of the undesired discharge state, thus degrading the maintainability of the apparatus and the mass producibility of thin films.
An object of the present invention is to provide a deposited film forming apparatus that can prevent intrusion of a plasma into a space between a base member and a power applying electrode due to deformation of the power applying electrode disposed above the base member in a vacuum chamber, and suppress occurrence of an abnormal discharge in the space.
In order to accomplish the above object, the present invention provides a deposited film forming apparatus comprising a power applying electrode disposed above a flat plate type base member grounded, in a vacuum chamber, and a power source for supplying a power to the power applying electrode, the apparatus being constructed to apply the power from the power source to the power applying electrode so as to generate a plasma in a discharge space between the power applying electrode and a substrate disposed in opposition to the power applying electrode in the vacuum chamber and serving as an electrode in a pair with the power applying electrode, thereby decomposing a source gas introduced into the vacuum chamber to form a deposited film on the substrate, wherein the power applying electrode is fixed to the base member with the power applying electrode being isolated from the base member.
According to the present invention, the power applying electrode is fixed to the base member while isolating the power applying electrode from the base member in the vacuum chamber, which suppresses the deformation of the power applying electrode due to the thermal expansion, the thermal expansion under plasma irradiation, the deposition of the thin film on the power applying electrode, and so on when the power is applied from the power source to the power applying electrode to generate a plasma in the discharge space between the power applying electrode and the substrate in the vacuum chamber. This improves the uniformity of the plasma and thus decreases the unevenness of the film forming rate of the deposited film to be formed on the substrate. Further, the suppression of the deformation of the power applying electrode permits the distance to be kept uniform between the power applying electrode and the base member, which prevents the intrusion of the plasma into the space between the power applying electrode and the base member and the occurrence of the abnormal discharge in the space. This prevents the waste of the source gas and the generation of polysilane powder, which can improve the maintainability of the deposited film forming apparatus and reduce the cost for formation of the thin film. These permit the thin film to be formed in a large area, which enhances the mass productivity of thin film devices such as solar cells or the like.
The apparatus of the present invention preferably has a mechanism for conveying the substrate, and this mechanism is preferably one for forming the deposited film while conveying the substrate in the roll-to-roll system.
The distance between the power applying electrode and the substrate (electrode-substrate distance) is preferably 5 mm to 20 mm.
The effect of the present invention becomes more eminent by employing either or some of these preferred configurations.
Specific fixing methods suitably applicable include (1) a method of fixing the power applying electrode to the base member with an electrically insulating, fastening member, (2) a method of placing the base member around the power applying electrode and fastening the power applying electrode by the base member, (3) a method of pinching and fastening the base member by the power applying electrode and a power introducing portion penetrating the base member in order to supply the power to the power applying electrode, and (4) a method of fixing the power applying electrode to the base member with an electrically insulating adhesive. The method (2) can be implemented by adopting a method of providing the base member with a depression, fitting the power applying electrode into the depression, and pressing the power applying electrode on every side with fastening members penetrating the base member. In the method (3), the power introducing portion also serves as a fastening member and it is also possible to provide a plurality of power introducing portions. Some of these methods may also be used in combination.
In the apparatus of the present invention, the power applying electrode is preferably fixed to the base member at the end portions of the power applying electrode. The term xe2x80x9cend portionxe2x80x9d as used in the specification and claims refers to a portion within 2 cm from the periphery of the power applying electrode. From the viewpoint of action, the sufficient condition is that the power applying electrode is fixed to the base member at a location where the deformation of the power applying electrode is suppressed well.
It is preferable to place an electrically insulating spacer between the power applying electrode and the base member.
By placing an electrically insulating spacer between the power applying electrode and the base member to keep the distance between the power applying electrode and the base member at a desired value as described above, it becomes feasible to suppress the intrusion of the plasma into the space between the power applying electrode and the base member and the occurrence of the abnormal discharge in the space well. For example, when the power applying electrode is fixed to the base member with electrically insulating screws or the like, by keeping the distance between the power applying electrode and the base member at a desired value by a spacer, it is possible to prevent shorts between the power applying electrode and the base member due a thin film deposited on exposed surfaces of the screws. The insulating spacer can be in the form of either a block or a sheet, and the spacer is desirably small one in order to reduce influence of emission of impurities or the like from the insulating material constituting the spacer.
Further, it is preferable to fill the space between the power applying electrode and the base member with an electrically insulating material.
The filling of an insulating material between the power applying electrode and the base member as described above perfectly prevents the intrusion of the plasma into the space between the power applying electrode and the base member and the generation of the abnormal discharge in the space. Japanese Patent Publication No. 63-3338 discloses coating an electrode with an insulating material to prevent dispersion of a plasma, but it is actually impossible to perfectly prevent the intrusion of the plasma, the abnormal discharge, and so on in the case where the electrode is merely covered with the insulating material and there still exists a space.
Further, it is preferred that the distance s [mm] between the power applying electrode and the base member satisfies the relation of sxe2x89xa6k/P, where s [mm] is the distance between the power applying electrode and the base member, P [Pa] is the pressure in the vacuum chamber during formation of the deposited film, and k is a constant of 1500 [Paxc2x7mm].
As described above, by setting the distance s between the power applying electrode and the base member in the range of sxe2x89xa6k/P, it becomes feasible to suppress the intrusion of the plasma into the space between the power applying electrode and the base member and the occurrence of the abnormal discharge in the space and to prevent shorts between the power applying electrode and the base member due to deposition of thin films on screws or the like for securing the power applying electrode to the base member. Further, it can prevent loss of the source gas and generation of polysilane powder.
The present invention also provides a deposited film forming method using the above deposited film forming apparatus.